JERUSALEM — The Israeli military launched a long-threatened ground assault to take over Gaza City after a night of intense bombing, sending many residents fleeing Tuesday from the devastated but still densely populated urban area that is home to hundreds of thousands of Palestinians.

The ground operation and the intensifying bombardment deepened the humanitarian crisis in the Gaza Strip, which has been shattered by a nearly two-year war that has killed tens of thousands of people and caused rampant hunger. Palestinians in Gaza City described scenes of panic as Israel pounded the area with heavy airstrikes that shook the ground. Local health officials said that more than 20 people had been killed and dozens more wounded.

“We are all terrified,” said Montaser Bahja, a former schoolteacher

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